1. Field of Invention
The invention concerns a device for the hot dip coating of metal strands, especially steel strip, in which the metal strand can be guided vertically through a tank that contains the molten coating metal and through an upstream guide channel, wherein an electromagnetic inductor is installed in the area of the guide channel, which, for the purpose of retaining the coating metal in the tank by means of an electromagnetic blocking field, can induce induction currents in the coating metal, which, in interaction with the electromagnetic blocking field, exert an electromagnetic force.
2. Description of the Related Art
Conventional metal hot dip coating installations for metal strip have a high-maintenance section, namely, the coating tank and the fittings it contains. Before being coated, the surfaces of the metal strip must be cleaned of oxide residues and activated for bonding with the coating metal. For this reason, the strip surfaces are subjected to heat treatments in a reducing atmosphere before the coating operation is carried out. Since the oxide coatings are first removed by chemical or abrasive methods, the reducing heat treatment process activates the surfaces, so that after the heat treatment, they are present in a pure metallic state.
However, this activation of the strip surfaces increases their affinity for the surrounding atmospheric oxygen. To prevent the surface of the strip from being reexposed to atmospheric oxygen before the coating process, the strip is introduced into the hot dip coating bath from above in an immersion snout. Since the coating metal is present in the molten state, and since one would like to utilize gravity together with blowing devices to adjust the coating thickness, but the subsequent processes prohibit strip contact until the coating metal has completely solidified, the strip must be deflected in the vertical direction in the coating tank. This is accomplished with a roller that runs in the molten metal. This roller is subject to strong wear by the molten coating metal and is the cause of shutdowns and thus loss of production.
The desired low coating thicknesses of the coating metal, which vary in the micrometer range, place high demands on the quality of the strip surface. This means that the surfaces of the strip-guiding rollers must also be of high quality. Problems with these surfaces generally lead to defects in the surface of the strip. This is a further cause of frequent plant shutdowns.
In addition, previous hot dip coating systems have limiting values in their coating rates. These limiting values are related to the operation of the stripping jets, to the cooling processes of the metal strip passing through the system, and to the heat process for adjusting alloy coatings in the coating metal. As a result, the maximum rate is generally limited, and certain types of metal strip cannot be conveyed at the plant's maximum possible rate.
During the hot dip coating process, alloying operations for the bonding of the coating metal to the surface of the strip are carried out. The properties and thicknesses of the alloy coatings that form are strongly dependent on the temperature in the coating tank. For this reason, in many coating operations, although, of course, the coating metal must be maintained in a liquid state, the temperatures may not exceed certain limits. This conflicts with the desired effect of stripping the coating metal to adjust a certain coating thickness, since the viscosity of the coating metal necessary for the stripping operation increases with decreasing temperature and thus complicates the stripping operation.
To avoid the problems associated with rollers running in the molten coating metal, approaches have been proposed, in which a coating tank is used that is open at the bottom and has a guide channel in its lower section for guiding the strip vertically upward, and in which an electromagnetic seal is used to seal the open bottom of the tank. The production of the electromagnetic seal involves the use of electromagnetic inductors, which operate with electromagnetic alternating or traveling fields that seal the coating tank at the bottom by means of a repelling, pumping, or constricting effect.
A solution of this type is described, for example, in EP 0 673 444 B1. The solution described in JP 50[1975]-86,446 also provides for an electromagnetic seal for sealing the coating tank at the bottom.
Although this allows the coating of nonferromagnetic metal strip, problems arise in the coating of steel strip that is essentially ferromagnetic, because the strip is drawn to the walls of the channel by the ferromagnetism in the electromagnetic seals, and this damages the surface of the strip. Another problem that arises is that the coating metal is unacceptably heated by the inductive fields.
An unstable equilibrium exists with respect to the position of the ferromagnetic steel strip passing through the guide channel between two traveling-field inductors. The sum of the forces of magnetic attraction acting on the strip is zero only in the center of the guide channel. As soon as the steel strip is deflected from its center position, it draws closer to one of the two inductors and moves farther away from the other inductor. The reasons for this type of deflection may be simple flatness defects of the strip. Defects of this type could include any type of strip waviness in the direction of strip flow, viewed over the width of the strip (center buckles, quarter buckles, edge waviness, flutter, twist, crossbow, S-shape, etc.). The magnetic induction, which is responsible for the magnetic attraction, decreases in field strength with increasing distance from the inductor according to an exponential function. Therefore, the force of attraction similarly decreases with the square of the induction field strength with increasing distance from the inductor. This means that, when the strip is deflected in one direction, the force of attraction to one inductor increases exponentially, while the restoring force by the other inductor decreases exponentially. Both effects intensify by themselves, so that the equilibrium is unstable.
DE 195 35 854 A1 and DE 100 14 867 A1 offer approaches to the solution of this problem, i.e., the problem of more precise position control of the metal strand in the guide channel. According to the concepts disclosed there, the coils for inducing the electromagnetic traveling field are supplemented by correction coils, which are connected to an automatic control system and see to it that when the metal strip deviates from its center position, it is brought back into this position.
In the realization of this principle, i.e., the concept of the traveling field inductor with correction coils, it was found to be a disadvantage that the inductors for inducing the electromagnetic traveling field must have a relatively large overall height due to the required field strength and electric currents and the laminated cores needed for this. The height of the inductor is usually on the order of 600 mm. This has negative effects on the height of the column of liquid metal in the guide channel.
To avoid this problem, WO 96/03533 A1 describes a device of this general type, which uses an electromagnetic blocking field to hold back the coating material and in which only one induction coil is used. The overall height of the inductor is thus relatively small.
However, as the metal strand passes through the guide channel, a disadvantage that arises is that the strand experiences a strong ferromagnetic attraction to the walls of the guide channel. To prevent this, the blocking field inductors in this well-known installation are operated with alternating current with a frequency higher than 3 kHz. This reduces the ferromagnetic attraction to a very low level, but it cannot be completely avoided. Another disadvantage is that strong heating of the metal strand occurs as it passes through the guide channel.